[GHC] #15056: Wrappers inlined too late

[GHC]

#15056: Wrappers inlined too late
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider these two modules
 {{{
 module Foo where

   test :: Int -> Bool
   test 0 = True
   test n = test (n-1)

   foo :: Foldable t => Int -> t Int -> Int
   foo n xs | test n
            = foldr (+) n xs
            | otherwise
            = n+7
 }}}
 and
 {{{
 module Bar where
   import Foo

   blam x = f 3 [1..x]
 }}}
 If we simply inline `foo` into `blam`, we get `foldr/build` fusion.  And
 that is exactly what happens if you compile `Foo` with `-fno-strictness`.

 But what actually happens is
  * When compiling `Foo`, strictness analysis does worker/wrapper for
 `foo`.
  * The wrapper currently gets an aciviation of "active only in phase 0";
 see `Note [Wrapper activation]` in `WorkWrap`.
  * So `foo` is ultimately inlined (actually its worker is inlined too) but
 too late for fusion to take place.

 This is bad: optimising `Foo` makes the overall runtime increase.

 I have seen other examples of this.  The general pattern is:
 * Without worker/wrapper, a function `f` may inline, which exposes
 opportunities for functions in its body to participate in rule rewrites
 * With worker/wrapper, no inlining happens until phase 0; and that is too
 late for the rules to fire.

 Obvious thought: allow wrappers from strictness analysis to inline
 earlier.  I'll try that.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15056>
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